The present invention relates to a full hydraulic power steering system for use in a motor vehicle, such as a forklift truck, which does not require a steering gear or a mechanical linkage.
Full hydraulic power steering systems are well known. For example, a system as shown in FIG. 1 is known. This system is disclosed by Nissan Technical Review No. 19, published by Nissan Motor Co., Ltd. in December, 1983 (see pages 170 and 171)
Referring to FIG. 1, this known system comprises a steering unit 102 (a valve) operated by a steering wheel 101 with a spoke 112, a steering servo actuator 104 (an actuator), a hydraulic fluid line 105 leading from the steering unit 102 to the steering servo 104, a hydraulic fluid line 106 leading from the steering unit 102 to the steering servo 104, a hydraulic fluid tank 108, and a hydraulic fluid pump 107. The pump 107 supplies hydraulic fluid to the steering unit 102. The steering unit 102 distributes the supply of hydraulic fluid from the pump 107 to the hydraulic fluid lines 105 and 106, selectively. This system is installed in a three-wheel forklift 109 which has a rear steered wheel 103 and a pair of front unsteered wheels 111. The steering servo 104 includes a pair of hollow rods 141 having their outer ends fixedly mounted on the vehicle body 109 and inner ends separated by a piston slidably received in a cylinder 142. The cylinder 142 is slidably along the rods 141 and has opposite ends through which the rods 142 extends out of the cylinder 142. The rods 141 are hollowed to establish hydraulic fluid passages connecting the hydraulic fluid lines 105 and 106 to portion inside the cylinder 142 divided by the piston. The cylinder 142 is dirvingly connected to the steered wheel 103 by a chain drive 110. In FIG. 1, small arrows along the hydraulic fluid lines 105 and 106 denote fluid flows through the hydraulic fluid lines when the steering wheel 101 is turned counter clockwise to turn the vehicle 109 to the left by steering the steered wheel 103 to the right. When the steering wheel 101 is turned to the left in order to steer the vehicle to the left, this movement of the steering wheel 101 causes the steering unit 102 to supply hydraulic fluid to the steering servo 104 in an amount that is in proportion to the steering wheel angle along a direction indicated by the small arrows. The hydraulic fluid supplied to the steering servo 104 flows through the hollow of one of the rods 144 into the inside of the cylinder 142. Since the rods 141 are fixed to the vehicle body 109, the cylinder 142 moves in accordance with the total amount of hydraulic fluid supplied, turning the steered wheel 103 via the chain 110 through an angle (a steered angle) that corresponds to the steering angle of the steering wheel 101.
In the conventional full hydraulic power steering system, however, since the steering unit 102 and the steering servo 104 are interconnected with the hydraulic fluid lines 105 and 106, only, and a steering gear or a mechanical linkage are not used, the amount of hydraulic fluid supplied to the steering servo 104 does not correspond to the steering wheel angle through which the steering wheel 101 is turned thereby to cause a difference therebetween if oil leaks within the steering unit 102. This causes a change in the relationship of the directional position assumed by the spoke 112 with that assumed by the steered wheel 103.
This is particularly a problem in the case where the steering wheel stays in a directional position slightly different from the neutral position even though the steered wheel is oriented in the straight neutral position. That is, the forklift travels straight forward against the intention of an operator if he relies only on the directional position of the spoke 112 on turning vehicle in the direction indicated by the position of the spoke 112.
The present invention aims at solving the above mentioned problem.